Two or three dimensional digital machine tool control



Oct. 31, 1

R. W. TRIPP TWO OR Tl-IREEI DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 13 Sheets-Sheet 1 D6 Xian! INPUT I85 fi X I COARSE merm. I83; I 0' M1206 I I I LINEAR FINE l To Fl6.3

" CONVERTER an" X .ooo|"- mPu'r 24 D2 FEED RATE FR .l ANALOG ,I l-

FEED I CONVERTER i INPUT Di RATE OF 57 CURVATURE CHANGE 1 l FR3 1 8 x(O-|) 1 s x(O-?) VARIABLE s' x(O-7) NVERYER GEAR RATIO f 8 X (0-7) INPUT 45 C, CURVATURE l 0mm. CURVATURE 00 CONVER D3 INPUT 44 SLOPE DIGITAL I00. To

' ANALOG ANGULAR e coNvERTER l INPUT D7 LINEAR Y l9l I00." FINE I 1 13 43o ROBERT w. TRIPP,

INVENTOR. J LINEAR COARSE CONVERTER Q BY Iooofl Y ATTO R N EY- Oct. 31, 1961 R. w. TRIPP 3,00 9

TWO OR THREE. DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 13 Sheets-Sheet 2 FR 0.0 r

4 8 R m v B O v T ll 3 G D u 4 G 1 11 D J l 7 q R" F. u 0 LM BR 2 A 2 G IR 6 V A ME 6 a m w R R ES m G MW M ML 0 m R 8 T E WWL E Z 6 n 6 mw e As N u N IRN m M A H w w A E WWW E l FR R 0 m wm wwww mm EWH X X X X WW IA O 3 2 O U C 88 C ROBERT W. TRIPP,

IN VENTOR.

ATTORNEY.

Oct. 31, 1961 R. w. TRIPP TWO OR THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 13 Sheets-Sheet 3 TO FIG. 5

my in TO FlG.l I I78 5 Z i x i I79 I80 i ROBERT W. TRIPP,

I INVENTOR. FR! r I i W i I I l TO F134 y ATTORNEY.

Oct. 31, 1961 R. w. TRIPP 3,007,096

TWO 0R THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 13 Sheets-Sheet 4 TO FIG. 3

| l I l ROBERT W. TRIPP,

INVENTOR.

ATTORNEY.

Oct. 31, 1961 R. w. TRIPP 3,007,096

TWO 0R THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 13 Sheets-Sheet 5 COARSE ROBERT W. TRIPP,

IN VENTOR.

As BY TO FIG.6

ATTORNEY.

Oct. 31, 1961 R. w. TRIPP 3,007,096

TWO OR THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 13 Sheets-Sheet 6 RELAY A .020- .000"

COARSE ROBERT W. TRIPP,

IN VEN TOR.

ATTORN EY.

R. W. TRIPP Oct. 31, 1961 TWO OR THREE. DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 15 Sheets-Sheet '7 ROBERT w. TRIPP,

INVENTOR.

T0 FIG. 5

ATTORNEY.

R. W. TRIPP Oct. 31, 196.1

TWO OR THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11. 1957 13 Sheets-Sheet 8 TO FIG] T0 FIG. IO

TOOL RADIUS AMP TO FIG. 6

ROBERT W. TRIPP,

INVEN TOR.

ATTORNEY.

Oct. 31, 1961 R. w. TRIPP 3,0

' TWO OR THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 l5 Sheets-Sheet 9 COAR SE MEDIUM COARSE COARSE COARSE ROBERT W. TRIPP,

IN VEN TOR.

ATTORNEY.

MULTI-TURN POT.

1961 R. w. TRIPP 3,007,096

TWO OR THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 v 13 Sheets-Sheet 1 298 g 10 AMP f5? 20 FINE 2O COARSE MEDIUM MULTI-QTURN POT.

M U LTl-TU RN POT.

ROBERT W TRIPP,

IN VEN TOR.

ATTORNEY.

TWO OR THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 R. W. TRIPP Oct. 31, 1961 13 Sheets-Sheet 11 s |HHH |U| N mfiblil m A A F ==A==A= u I I I I 4--- 4:1: ua TL 08E @wc m9. w @E T {L Mom D mi .1 ||||B|| Ila IL A -4 -J p m All x m 30 6 A 3 u L n Al fi n w W m i liL m m kmw I R E -v m9. Hui m 3; SE

ROBERT W, TRIPP,

INVENTOR.

ATTORNEY.

Oct. 31, 1961 R. w. TRIPP 3,

' TWO 0R THREE DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 15 Sheets-Sheet 12 RESU LTANT ERROR e PRESENT CARRIAGE POSITION l P2 A 9 COORDINATE SYSTEM 2 z (COORDINATE AY A's x' AXIS SYSTEM Y' AXIS I 11 I A2 I AS' M2 I A'Al 1 COORDINATE SYSTEM 3 COORDINATE AS AAZ SYSTEM 2 AAZ IN V EN TOR.

ATTORNEY.

AAY

ROBERT W. TRIPP,

Oct. 31, 1961 R. w. TRIPP 3,007,096

TWO OR THREE. DIMENSIONAL DIGITAL MACHINE TOOL CONTROL Filed Sept. 11, 1957 13 Sheets-Sheet l3 g .21 --cos9 cosd) X -sm Guns 4) Y xi $9 20 INPUT DIGITAL 9 TO TL 2 (5m 4 ANALOG 4 CONVERTER e+ (SIN-9 cos 9) 4 Y INPU l-X-l (cos Gcos DIGITAL 4) ANALOG 5 CONVERTER ROBERT w. TRIPP, INVENTOR.

ATTORNEY.

United States Patent 3 007,096 TWO on THREE onwENsroNAr. DIGITAL MACHINE TOOL CONTRGL Robert W. Tripp, Bronxville, N.Y., assignor to Inductosyn Corporation, Carson City, Nev., a corporation of Nevada Filed Sept. 11, 1957, Ser. No. 683,404

- 38 Claims. (Cl. 318-162) This invention relates to two or three dimensional digital machine tool control and in general to the two or three dimensional control of machine tools in which the rotating cutter of finite radius and a workpiece are moved relatively to each other according to a program of two or three dimensional information supplied to the traversing drives of the workpiece carriage or cutting head in the machine tool in order to generate on the workpiece a surface or profile of specified shape.

The present invention relates to an automatic machine control system for a continuous machining operation, or contour milling. This system employs the Inductosyn (Pat. No. 2,799,835) as the data element.

'The Inductosyn has very significant advantages as the data element for contour milling in comparison with other types of data devices. In addition to its extremely high accuracy, it permits a great simplification of the preparation of a program, provides a constant zero reference, and permits accurate checking of selected points with respect to this reference, and eliminates accumulation errors.

For the general case of cutting an arbitrary curve, the input data consists of selected coordinates along the curve to be cut. Following the data input device is a unit which may be referred to as a computer, which accepts the input data and converts it into rate drive commands for the machine drives. The machine parts then move .along separate axes simultaneously at continuously-changing rates and the required curve is cut in the work-piece.

In general, the commands to the machine drives must be the rates and distances which the machine parts or carriages themselves must move in order to cut the desired curve. Nearly all machining operations utilize circular or spherical cutters, and in this case the path of a fixed point on the machine carriage (the cutter center) is not the same as the actual curve to the cut in the work-piece. It is a parallel curve, which is displaced along the normal to the work-piece by the amount of the cutter radius.

In automatic contouring systems, other than the present type of system, this cutter center path which must be computed and introduced as the data input to the control system is computed as an integral part of the command data input, This computation is not a simple one, since in general it is not possible to write a single equation for the required curve. There is the further disadvantage that any input program prepared in this way is valid for only one size of cutter, in one particular state of sharpening, and if the cutter is dressed or a different cutter used, an entirely new program must be prepared. In contrast, "because of the Cutter Radius Corrector, which can be realized with the Inductosyn, the present control system utilizes a program containing information on the actual curve to be .cut in the work-piece, and correction for the actual radius of the cutter being used is made directly at the machine. If necessary, the correction can even be changed to compensate for cutter wear as the work proceeds.

Another advantage of the present system, is derived from the fact that, while the input data of the system 1 Trademark.

3,007,096 Patented Oct. 31, 1961 are in digital form, the actual machine drive control is an analog type of system. This results in the cutting of a continuous, smooth curve in the work-piece, in contrast to the succession of short chords or arcs which results from systems in which the digital form of data is carried all the way to the machine drive itself.

Since the command signals to the automatic contouring machine are in the form of rates, such system is inherently subject to cumulative errors resulting from the integration of small rate errors over the length of the curve being cut. In the present control system, provision is made for accurately checking the position at selected points along the curve, in advance of arrival of the cutter at that position; the error is computed and corrections made to the feed rates so that the curve will accurately pass through each of these check points.

The features of checking and correcting the integrators and the tool position for points in a plane, namely for the X and Y axes are described and were formerly claimed in Case 4, SN. 563,125, filed February 2, 1956, by R. W. Tripp for Automatic Digital Machine Tool Control. The instant case is a continuation of Case 4 and in addition discloses and claims the above features not only the X and Y axes but also for the Z axis.

Mechanical integrators, which may be of the balldisk-cylinder type, are employed for integrating the curvature and rate of change of curvature signals with respect to the feed rate to obtain a slope output for combination with the output of the slope converter, providing an input for the feed resolver. The output of the feed resolver has a similar type of integrator for integrating the sine and cosine functions of the slope angle with respect to the feed rate to provide a shaft output for the coordinate tool drives. -It is expected that this type of integrator will depart from ideal about one part in 1000 or less. This error is prevented from accumulating to values in excess of the tolerance. Thus, if one desires to machine a part with an accuracy of about .001 inch it will be necessary to check the progress of the curvatureintegrator and the integrators for the servo drive, as well as the tool position about once every inch of total motion.

The present case provides a control system to make this check, compute the error if any, correct the slope angle curvature and rate of change of curvature controls in accordance with their respective input data and also to correct the tool position without stopping the machine.

As a basis for comparison and to determine the error in the tool position, the input data includes linear position data of discreet check points such as one inch apart along the tool path. This data may 'be referred to as primary data and it is decimal digital data in terms of length, a converter being provided as described and claimed in copending application S. N. 540,429, filed October 14, 1955, by Robert W. Tripp for Automatic Machine Control, Case '1, now Patent 2,849,668, dated August 26, 1958. 'As disclosed in said Case 1, coarse, medium and fine data elements are provided to obtain a large range of values with high accuracy, a potentiometer being provided as the coarse data element, a rotary resolver or position measuring transformer being provided as the medium data element and linear Inductosyn being provided as the fine data element. The decimal digital linear input data is converted into sine and cosine signals for supply to the quadrature windings of and medium and fine 'data elements, this difference being noted that in Case 1 the quadrature windings of the corresponding fine data element are on a linear element, whereas in Case 4 and in the present invention such windings are on a rotary element. The linear form is used elsewhere in the system. The coarse data element has an accuracy of about one inch in 1000, the medium data element has an accuracy of about .01 inch in 10, while the fine data element has an accuracy of .0001 inch in .10 inch. Thus the percentage accuracy of: each data element is about the same of one part in 1000. These coarse and medium data elements are further described above under the paragraph heading Inductosyn.

Position checker.-In the two dimensional case, as the machine approaches the check point specified by the X and Y input information, the difference between the ideal position as called for by the primary input data and the actual machine position is continuously measured, the actual machine position being dependent on the command data of slope, curvature and rate of change of curvature. These differences will be the X and Y components of the distance from the check point to the actual machine position. These two diflYerences are applied to the two windings .of a resolver connected to the slope angle shaft. The output from this resolver will then be the resultant distance to the present machine position from the check point if everything has been operating perfectly and the machine is heading directly towards the check point along the present slope angle. If the machine is going to miss the check point there will be another output from the fourth winding of the resolver that is a measure of the distance by which the machine is going to miss the check point. This output is then re-resolved into x and y components and used to correct the X and Y integrator outputs to make sure that the machine does go through the check point accurately. From the above it can be seen that this checking information exists and is useable even while the machine is moving and therefore the checking operation can be done without stopping at the check point and correcting. This is highly desirable because a mark is always left by the tool when the machine is stopped with the tool still cutting. Case 4 and the present invention provide for keeping the tool moving with respect to the workpiece at all times to obtain a good finish on the machined surface.

The above position chec also provides the checking moment for the curvature and slope angle checkers. This is done by observing the output of the resolver that 'is the resultant distance from the machine position to the check point. When this output goes through zero, that is, when the distance measured goes through zero, is the exact moment when the curvature, slope angle and position must be correct. However, since a finite time is required to make a correction, the correction is actually started a short distance in advance of this zero. However, the rate of change of curvature is read in precisely at the time that the distance is zero since the change is essentially instantaneous. This moment when the distance measured is zero is also used to tell the punched tape or card reader to advance and read the next piece of information.

The end point of one segment is the same as the starting point of the next and is the check point. However, the correction starts just prior to the end of the segment and should be complete when the end point is reached; The check points and the ends of the segments coincide.

Normally x, y positions, slope, curvature and rate of change of curvature are supplied as the initial starting conditions. Rate of change of curvature is changed stepwise as required at each junction point between two successive segments. The other variables change continuously through the segment as a result of successive integrations.

One distinction between rate of change of curvature and the other variables is that rate of change of curvature is held constant during a segment and if changed must change discontinuously at the check point while the other variables would have the new values at the check point if the integrations were free of error.

It should be noted, however, that the position, slope and curvature can obtain their values from two sources; (1) the aforementioned integration outputs of the X and Y integrators, (2) by differential input from their respective drive motors. Corrections are made starting shortly before a check point to differentially add in such values as are required to make the values agree with the corresponding values inserted at the check point.

Curvature and slope angle checkers.The curvature and slope angle data are the values that these quantities should have at the time that the machine reaches the point called for by the X and Y position information. Thus the checking operation is done only as the machine passes through or very near the checking point specified by X and Y. This moment is determined in a manner described in the section on Position Checker. At the moment of checking, the present curvature in the machine is compared to the desired curvature given on the punched tape or card. If there is an error, it is arbitrarily corrected by the correction servo driving though the differential in the input to the curvature integrator. The resultant slight change in the curvature of the machine part will not be noticeable. Similarly at the moment of checking mentioned above, the present slope angle in the machine is compared to the desired slope angle specified on the punched paper tape or cards and any difference arbitrarily corrected. This may result in a slight change in the slope angle of the machine part, but this change will be only about 1 part in a thousand of the possible slope, so it will be a very slight change.

It will be noted that the X and Y position information is used only at or near the time when the machine is going through the check point. Similarly' the curvature and slope angle information is used only at this time. Thus it is rather simple to read the information for these quantities into the machine during the time when the last information was used and before the next information is needed.

The present invention checks the slope angle and the curvature controls for the input to the feed resolver, and also check the tool position and operate the checking and the card advance in timed relation with each other.

As noted above under the heading of Feed Rate, the X and Y drives are not prevented from operating during the transitory stage when there is an advance from one set of slope or curvature data to another. Case 3 (Patent 2,875,390, dated February 24, 1959) discloses switch PB2 in FIG. 15 in circuit with the servo control for the curvature integrator and a corresponding switch FBI in FIG. 16 for the servo control for slope angle, both of these switches serving to disconnect the feed rate until the slope angle and curvature values as set up in their controls correspond to the values called for by their respective input data. According to the present invention, the switches FBI and PB2 of Case 3 are operated automatically by the position checker, the feed rate drive being always in operation.

Case 241e, Ser. No. 608,024, filed Sept. 5, 1956, by Robert W. Tripp for 3-Dimensi0nal Control Method and System Case 241e, now Patent 2,843,811, dated July 15, 1958, describes and claims the extension of the 2-dimensional tool control of Case 3 to the 3-dimensional axes X, Y and Z. This Case 241a also describes and claims the component solver shown herein in FIG. 3 for the feed rates on X, Y and Z axes. Case 241s does not include a specific disclosure of or claim either tool radius correction or position checking.

Case 241g, Ser. No. 608,357, filed Sept. 6, 1956, by Robert W. Tripp for "3-Dimensional Tool Radius Computer Case 241g describes and claims the tool radius computer of Case 5 (S. N. 561,769, filed January 1, 1956) for the 3-dimensional axes X, Y and Z and is thus the combination of Case 24le (Case 3 extended to 3D) and Case 5 extended to 3-D.

Zero ofiset and program advance control are covered by Case 3 and Disc. 264," SN. 638,722, filed February 7, 1957, by R. W. Tripp for Zero Olfset for Machine Tool Control. I

Present invention A number of features used in the present invention have already been described in connection with some of the co-pending cases referred to above. In general, the present invention is a combination of Case 241g which covers the tool radius computer 3-D, with improved apparatus for position checking for both 2-D and 3-D.

The present invention provides for maintaining the tool in motion during checking and correcting the integrators and the tool position. This is accomplished by controlling the servo drives for the tool or driven element in accordance with the sum of (1) the tool radius offset signal (and zero offset if any), (2) the command shaft input, namely the output from the x and y integrators, is accordance with the command input data of slope, curvature and rate of change of curvature integrated with respect to the feed rate, and (3) a servo motor driven shaft input controlled in accordance with the error if any in the tool position, that is in accordance with the difference between the ideal tool position as designated in the coarse, medium and fine increments by the primary input position data and the actual tool position resultingfro m the action of the command data. These shaft and electrical inputs are added in coarse, medium and fine transmitters for transmission to coarse, medium and fine receivers which control the machine servo drives. The present invention provides shaft and electrical inputs for accomplishing these features not only for 2-D but also for 3-D, by taking into account the angle which represents the angle that the tool path is or extends above or out of the X, Y plane. This is done by providing linear input data of an ideal or check position on the ideal path, with reference not only to the component thereof in the plane of two of the axes, but also with reference to the path component out of that plane. The invention also provides means for translating such check data both in the plane and out of the plane into check signals and it also provides means responsive to the 3-D feed rate drive and such 3-D check signals to compute the error of the amount by which the machine drives would miss the check point if they continued at the slope determined by the angles and The invention further provides means for correcting the 3-D feed rate or control drives in accordance with the error thus computed.

In certain prior practice it has been proposed to provide command data which sets up a count which is stored, the machine itself producing another count which serves to operate the machine drives until the stored count is reduced to zero. According to another proposal, an initial cut is made on the work-piece, its error computed, and another cut rnade accordingly. The present inven- 'tion differentiates from prior arrangements of the types just described, with resulting increase in simplicity, flexibility and accuracy by taking advantage of the highly accurate performance of an Inductosyn or similar fine data element in the following way. The command data is translated into a control feed rate drive, and in so doing, an unwantederror occurs due to the integrators employed in this translation. The feed rate drive, in accordance with both the :0 and slopes is located ahead .of the machine, and in turn this feed rate drive is translated into the controlled command drives for the machine parts. As the error is thus located ahead of the machine in the control feed rate drive, being also corrected by the check data at that point which is likewise ahead of the machine, the command input data is translated into analog form, corrected for error, ahead of the machine 'and' hence the corrected command feed rate drives represent in analog form a highly accurate drive for the machine part-s. The accuracy of such corrected feed rate drives is maintained through the use of a plurality of grades of data elements one of which is a fine grade such as the Inductosyn, for translating the corrected and accurate feed rate drive into the controlled serve or command' drive for the machine parts.

In thus providing 9 and slope controls for the control feed rate drive, such drive makes it possible to employ a separate 3-D tool radius control having an input in accordance with the feed rate drive as corrected by the input check data.

For further details of the invention reference may be made to the drawings wherein, FIG. 1 is -a diagram illustrating primary and command data inputs for coordinate axes X and Y and feed rate and associated converter or computer apparatus for operating the 0 and feed rate shafts and for supplying linear data in X and Y, in fine, medium and coarse increments.

FIG. 2 is a diagram illustrating primary and command data inputs and associated converter or computer apparatus for operating a shaft in accordance with the angle at and for supplying linear input information of a third coordinate axis Z in coarse, medium and fine increments.

FIG. 3 is a diagram showing schematically in perspective a component solver having inputs of 6 and 0+ and having outputs of sin 0 cos 4) and cos cos 0 with integrators for integrating the feed rate of such component.

FIG. 4 schematically illustrates a resolver having an input 41 and a feed integrator therefor.

BIG. 5 shows schematically a portion of the system receiving the feed rate values of x and y from FIG. 3 and 0 from FIG. 1 with a position checker therefor and supplying corrected values of x and y via FIG. 7 to FIG. 9.

FIG. 6 shows a position checker receiving values of the feed rate Z from FIG. 4 and values of 5 'via FIG. 4 from FIG. 2, FIG. 6 supplying the position checked values of Z via FIG. 8 to FIG. 10.

FIG. 7 shows a tool radius correction for x and y supplied to the system of FIG, 9 for combination with the tool radiuscorrection of FIG. 5.

FIG. 8 schematically shows the tool radius correction portion of the system receiving values of 4; via FIGS. 6 and 4 from FIG. 2 and supplying tool radius correction to FIG. 10.

FIG. 9 shows the servomotor control for the X and Y machine elements, the positions of which are thus determined by both the position checker and tool radius correction.

FIG. 10 schematically shows the Z machine element which is also controlled in accordance with the tool radius correction and with the position checked value of Z via FIG. 8 from FIG. 6.

FIG. 11 is a key sheet showing how the odd numbered FIGURES 1 to 9 fit in sequence with the even numbered FIGURES 2 to 10 in sequence there below to provide a 2-dimensional and 3-dimensional digital machine tool control according to the present invention.

FIG. 12 is a perspective I l-dimensional coordinate view andillustrates-the computation of the error of the tool or carriage position in the 3-dirnensional case and for which four resolvers may be employed as described in connection with schematic showings in FIGS. 14, 16, 17, 18 and 22.

FIGS. 13, 15 and 19 are schematic views useful in connection with FIG. 12.

FIG. 20 is a schematic diagram showingthe tool path 1, its component in the X, Y plane and its angles 0 and o and its components on the X, Y and Z axes.

FIG. 21 is a schematic block diagram illustrating the mathematical operations performed on the shaft rotation outputs of FIGS. 1 and 2, with the use of the component solver and adder of FIG. 3 for producing shaft outputs characteristic of the X, Y and Z components of the tool path.

Referring in detail to the drawings, as shown in FIG. 20,

the tool path 1 is illustrated with reference to the three dimensional coordinate axes X, Y and Z, the angle representing the angle between the X axis and the tool path component 2 in the X, Y plane while the angle represents the angle that the tool path 1 is or extends above or out of the X, Y plane. The invention provides means for supplying analog values of 0 and S and means for resolving these angles into their components X, Y and Z according to the following formulae, the length of the tool path being taken as unity.

X =cos 0 cos Y=sin 0 cos Z=sin 4) The term sin 4 or Z is solved with the resolver R1 in FIGS. 4 and 21, while the terms cos 0 cos and sin 6 cos are solved by the component solver R2 in FIGS. 3 and 21. As shown in FIG. 21, the component solver R2 has as inputs the value 0 and also the value 0+ obtained from the adder 3, FIGS. 3 and 21, which has both 0 and as inputs.

By integrating the feed rate with values proportional to the components above described and shown in FIG. 20, the tool or other driven element is caused to follow a path in space in accordance with digital input data appropriate to its angles 0 and (p. The X and Y machine drives are indicated in FIG. 9 while the Z machine drive is indicated in FIG. 10, as will be described in detail later.

The above general statement of the matter is given at this point in order to describe the invention in general terms, and in connection therewith the following general description may also be considered.

The present case describes and claims three basic parts as follows:

(1) The command unit of FIG. 1 which determines continuously varying values of angle 0 at shaft 4 from decimal, digital inputs D3 of slope, D4 of curvature, D5 of rate of change of curvature and D2 of feed rate.

(2) The resolving unit which operates on the values of angle 0 and values of the feed rate to determine the X and Y coordinates in terms of the angular position of the shaft corresponding to shaft 4 in FIG. 1.

(3) The driving unit similar to present FIG. 9, which converts the X and Y shaft instructions to coarse, medium and fine electrical signals which in turn cause the machine elements to servo to the correct positions.

Generally speaking, the two-dimensional case has been extended to three dimensions as disclosed and claimed herein in connection with tool radius correction and position checking by making the following improvements:

(1) Command unit.The command unit includes not only the command unit of FIG. 1 as described above for obtaining continuously varying values of angle 0 at shaft 4, but it also includes, as shown in FIG. 2, decimal digital values and inputs D8 of slope, D9 of curvature and D of rate of curvature change and digital-to-analog converters controlled thereby for obtaining continuously varying values of angle at shaft 5.

(2) Resolving unit.As above described in connection with FIG. 20, taking the length of the tool path as unity, its component Z=sin 6 is obtained with a conventional resolver R1 in FIGS. 4 and 21, while its other components X=cos 0 cos and Y=sin 0 cos are obtained with the resolver R2 in FIGS. 3 and 21. While a detailed description of resolver R2 will be given later, at this point it may be noted that this resolver R2 is a combination of three devices, namely;

a. A sine-cosine mechanism.

b. A planetary diiferential, see FIG. 3, in that the outer frame 6 is driven about its axis at angle 0 (by pinion 7 which drives gear 8 on frame 6) frame 6 having a ring gear '9 having inwardly extending teeth 10 which mesh with the teeth 11 on planetary gear 12 which rotates about its axis and having a rotary support 13 at the outer end of a crank 14, the inner end of crank 14 being fixed to shaft 15 which rotates on the axis of frame 6 at angle 0+. The sum of 0 and (15 is the output of adder 3 in FIG. 21 and also FIG. 3, the latter showing this adder as a differential gear unit having inputs of 6 from shaft 4 in FIG. 1 and from shaft 5 in FIG. 2.

c. A resolver, in that the sliders 16 and 17 have slots 18 and 19 of a Scotch yoke mechanism 20 applied to the crank pin 21 on gear 12 which rotates inside of ring gear 9, see FIG. 3.

(3) Driving unit.-In addition to the drives for the X and Y machine elements as in FIG. 9, the invention adds a drive for the Z machine element as in FIG. 10.

The invention will be described in further detail under the following headings, which represent various components of the machine control method and system; the command unit of FIG. 1, command unit of FIG. 2, component solver of FIG. 3, resolver of FIG. 4, the position checker for X and Y in FIG. 5 and for Z in FIG. 6, tool radius correction for X and Y in FIG. 7 and for Z in FIG. 8, the X and Y driving unit of FIG. 9 and the Z driving unit of FIG. 10. Before taking up these headings, a description will be given of the feed rate as this forms an input to FIGS. 1 to 4 inclusive.

FEED RATE In FIG. 1, the input DZ supplies a decimal digital input of feed rate to the analog feed converter 24 which supplies a voltage as disclosed in SN. 557,035, now Patent 2,875,390, for comparison with the voltage of tachometer 25 driven by feed rate motor M1. The servo indicated at 26 drives the motor M1 at such a rate that the difference between the voltage generated by the stepping switch conversion circuit, not shown, of the converter 24 and the tachometer 25 is essentially zero.

The feed rate motor M1 drives the feed rate shaft FR which in FIG. 1 is also an input indicated at FR3 to the variable gear ratio VGl, described later and also an input indicated at PR4 to the ball-disk-cylinder integrator BDC1, described later.

As shown in FIG. 3, the feed rate FR is also an input indicated at FR40 to the ball-disk-cylinder integrator, BDC2 and an input FRS to the ball-disk-cylinder integrator BDC3, these integrators, as later described, being controlled by the sliders 16 and 17 of resolver R2, pertaining to the X and Y machine elements.

As shown in FIG. 4, the feed rate PR is an input PR6 to the ball-disk-cylinder integrator BDC4 in the output of resolver R1 and pertaining to the Z machine element.

As shown in FIG. 2, the feed rate PR is also an input PR7 to the variable gear ratio VG2 and an input PR8 to the ball-disk-cylinder integrator BDCS later described.

COMMAND UNIT OF FIG. 1

In FIG. 1, the slope data D3 represents a decimal number in terms of angles, the curvature data D4 represents a decimal number in terms of the reciprocal of radius and the rate of curvature change data D5 represents a number in terms of speed, the speed number, as described and claimed in S.N. 557,035, now Patent 2,875,390 being in a system of numeration having a radix of 2 to the Nth power, where N is an integer here shown as 3, the system being octal.

The slope 0 of the component 2 in the X, Y plane of the tool path 1, see FIG. 20, depends upon the ratio of the feed rates of the corresponding X and Y machine elements of FIG. 9. This ratio is established with a single datum of input information D3. This is accomplished by positioning the shaft 4 in FIG. 1 in accordance with the slope data D3 and by resolving the angular position of the feed rate resolver R2 in FIG. 3 into cofunction'controls in space quadrature, by operating the ball slides 27 and 28 of resolver R2 as inputs for the integrators BDC2 and BDC3 to establish the feed rates at shafts S11 and S12, FIG. 3, to establish the feed rate ratio on the X and Y axes.

The resolver shaft position 0 is established from input information D3 of slope angles expressed in terms of angles on a decimal basis, a digital-to-analog converter 44 being provided to convert this inputto the angular position of shaft 4 as described and claimed in copending application S.N. 540,748, filed October 17, 1955, by'R. W. Tripp for Automatic Shaft Control now Patent 2,839,711, dated June 17, 1958, and assigned to the assignee of the present application, that application also disclosing and claiming a computer for computing the sine and cosine values of an angle equal to the sum of the angles represented by the digits in decimally related digital groups as indicated by the input D3. Said application also discloses producing the co-function sine and cosine values of the angle in coarse and fine increments, the coarse increment being supplied to the medium resolver 29, the fine increment to the Inductosyn 30. For example, the coarse increment of sine 0 may be supplied to winding 31, the coarse increment of cosine 0 to winding 32, windings 3 1 and 32 being in space quadrature and inductively related to the relatively rotatable winding 33. having a driving connection as indicated at 34 to the relatively rotatable winding 35 of Inductosyn 30. The fine increment of sine 0 may be supplied to Winding 36, the fine increment of cosine 0 to winding 37. Windings 36 and 37 are inductively related to the relatively rotatable winding 35, the latter having a driving connection indicated at 38 to gear 39 of differential gear DGl. Gear 39 is connected by gear 40 to servo motor 41 having an amplifier 42 and controlled by a well known synchro switch 43. Motor 41 provides a shaft input to the differential gear D61 and operates it to thereby operate resolver 2% and Inductosyn 30, in turn, to reduce to zero the error current in windings 33 and 35, whereby shaft 4 is driven to an angular position or to continuously varying positions in accordance with the data D3.

The circuit of motor 41 is controlled by a switch S30 later described.

As described in the above mentioned patent application SN. 557,035, the ratio of the speed rates of the driven elements on the X and Y axes is changed, as required for a circular path, i.e., part or all of a circle, with a single datum of curvature input information D4. The input D4 thus provides curvature input information on a decimal basis in terms of curvature (reciprocal of radius) and the converter 45 converts this digital data to an analog value expressed as a shaft speed for addition to the position of shaft 4 as determined by the slope control D3.

As described and claimedin S.N. 557,035, the differential gear D62 has a spider having an output shaft S driven at a speed equal to the sum of the speeds of shaft S3 from the rate ofcurvature change and the speed of shaft S2 driven by servo motor M2. The shaft S5 is a part of the spider and it has a driving connection 46 with the slider 47 of a potentiometer 48, the servo circuit including motor M2 and amplifier 49 driving the shaft S2 and hence gears 50 and 51 and gear 52 to a position or at a speed which reduces to zero the error current determined by the difference between the potentials established by the position of slider 47 and the curvature instruction from converter 45, as set up in the input D4.

S.N. 557,035 refers to page 12 of reprint from Machine Design, August 1945 through February 1946, entitled Designing Computing Mechanisms by MacomFry, for a description of the differential gear like D61 and DG2 and elsewhere; also page 30 thereof for the well-known integrator like BDCl.

Switch S is similar in function to switch S30, to render its servo motor M2 inactive at certain times as described later.

The shaft thus in part at least is driven to a position or at a rate dependent upon the curvature instruction in the input D4. Shaft S5 operates gear 53 which operates the ball slide 54 to integrate the feed rate drive 1.0 PR4 accordingly, the output S1 being added through gears 55 and 56 to the shaft 4 through the differential gear DG1.

As described and claimed in S.N. 557,035, the rate of change of curvature input data D5 is converted into analog form to provide a position or continuously varying speed values of shaft S3 which is added through differential gear DGZ to the position or speed of shaft S5, whereby the curvature instruction in shaft S5 is thus modified in accordance with the rate of curvature change instruction in the input D5. Application S.N. 557,035 points out certain advantages in having the input D5 in octal form as indicated with its conversion by converter 57 to binary form, to operate gears VGl in different combinations to change the speed of the feed rate input PR3 into the speed of the output shaft S3 in accordance with the instructions set up in the input D5.

Hence the 0 shaft 4 in FIG. l is controlled by the combined effect of the instructions in all of the inputs D3, D4 and D5, whereby the combined effect of all of these instructions may be resolved into co-function space quadrature feed rates for the X and Y drives.

Application S.N. 557,035 refers to pages 31 to 33 of the above publication Designing Computing Mechanism for a description of the principle of operation of the binary gear device VG1, although said application discloses and claims an improved construction. Said application also refers to Equation 7, page 8, Vol. 27, Radiation Lab. Series, published 1948 by McGraw-Hill Book Co., said equation pertaining to the speed of the output shaft of a spur gear cell in relation to the spider speed and the input shaft speed, a number of such cells being useful for operation by the binary instructions supplied to the variable gear ratio VG1.

COMMAND UNIT OF FIG. 2

Referring to FIG. 2, the circuit here shown is similar to the circuit in FIG. 1, the slope input D8, the curvature input D9 and the rate of curvature change D10 corresponding to the inputs D3, D4 and D5 respectively. The circuits and devices controlled by the inputs D8, D9 and D10 are also similar to the corresponding items in FIG. 1, with this main difference, that the inputs D8, D9 and D10 have values appropriate to positioning or driving the shaft 5 at the angle qb, appropriate to the desired motion of the machine in the Z direction, see FIGS. 10, 20 and 21.

Accordingly, the slope data in the input D8 is converted by converter 60 into coarse and fine increments of sine and cosine values by the coarse resolver 61 and the Inductosyn 62 which are driven by the servo motor 63, under control of synchro switch 64, to reduce the error current to zero, as previously described, to thereby drive shaft 5 through differential gear DG'3 as called for by the slope input D8. The position or rate of shaft 5 is varied by the curvature input D9 acting through the digital-to-analog converter 68 and servo motor 65, differential gear DG4, shaft 66, ball slide 67 to integrate the feed rate FRS and provide a shaft output S6 which is added through differential gear DG3 to the shaft 5. Also, the curvature shaft output S6 is modified in accordance with the rate of curvature change instruction in the input D10 through the addition of the shaft output S7, from variable gear ratio VG2, through differential gear DG4, to shaft 66 and the input of ball slide 67 to the integrator BDCS. The input D10 controls the digital-to-analog converter 69 which controls the variable gear ratio VGZ having the feed rate input PR7.

The servo circuits of motors 63 and 65 in FIG. 2 are controlled by switches S8 and S9, as in FIG. 1, and later described.

The output of shaft 5 is thus in accordance with the combined instructions in the inputs D8, D9 and D10.

COMPONENT SOLVER OF FIG. 3

The terms cos 0 cos (X) and sin 6 cos p (Y) are solved by the component solver R2 in FIG. 3.

The planetary gear 12 is so mounted that it will rotate about its center 23 while being driven by shaft 15 through crank 14. Gear 12 meshes with ring gear 9, its pitch diameter being equal to /2 that of ring gear 9. Pin 21 is integral with gear 12, and is located on the pitch line. It drives the Scotch yoke having yokes or sliders 16 and 17. Ring gear 9 is itself driven about its axis 22 by pinion 7 acting through gear 8. The distance of pin 21 from axis 22 will be referred to as R.

The component solver R2 is a combination of the following three devices.

(1) As a planetary diiferential, if the center 23 of gear 12 is rotated about axis 22 by angle a and if ring gear 9 is rotated about its axis 22 by angle 0, then planetary gear 12 will rotate about its own center 23 by angle a0.

(2) With ring gear 9 fixed, as planetary gear 1'2 is rotated about its center 23 by an angle 3, pin 21 will proceed in a straight line across the diameter of ring gear 9 in such a Way that its distance R from axis 22 is proportional to cos It can be seen that with ring gear 9 free to rotate, this proportionality still holds, with respect to ring gear 9.

(3) As a resolver, if ring gear 9 is rotated about its axis 22 at an angle 6, then pin 21 will cause yokes or sliders 16 and 17 to move proportionately to R cos 0 and R sin 0.

By combining the above three modes, output yokes or sliders 16 and 17 can be caused to move proportionally to sin 0 cos 5, and cos 6 cos as follows:

(a) Revolve center 23 about axis 22 through an angle 0+, by turning shaft 15. Shaft 15 is operated by the sum of angle 0 from FIG. 1 and 5 from FIG. 2, these values being added in the differential gear or adder 3 which supplies the sum 0+ as an output for shaft 15.

(b) Rotate ring gear 9 through angle 0, by turning gear 7, angle 0 from FIG. 1 being an input to gear 7.

(c) By differential action, planetary gear 12 will rotate about its center 23 at an angle (it-*0, where a=0+, namely at an angle 0+0 or angle Therefore, pin 21 will move along a diameter of ring gear 9 proportional to cos or R=c0s But ring gear 9 has been rotated through angle 0. Therefore, by resolver action, yokes or sliders 16 and 17 move amounts proportional to R sin 0 and R cos 0, or sin 0 cos qb, and cos 0 cos 4:, respectively, since R=c0s g5.

As above described, the ball slides 27 and 28 are actuated by the slides 16 and 17 respectively to integrate the feed rate FR40 and FR5 respectively supplied to the respective integrators BDCZ and BDC3, whereby the shafts S11 and S12 are driven at rates corresponding to the X and Y components of the tool path.

RESOLVER OF FIG. 4

As above described, the angle instruction of shaft 5 from FIG. 2 is resolved by resolverRl and its Scotch yoke slider 70 into a linear movement proportional to sin slider 70 actuating the ball slide 71 of the integrator BDC4 which has the feed rate input PR6, to provide a shaft output S13 carrying a feed rate instruction in accordance with the Z component of the tool path.

POSITION CHECKER FOR X AND Y A description will now be given of the determination of the machining errors of a machine tool control system. The two dimensional case is disclosed and claimed in the present case which uses resolvers in the usual unilateral manner. The bi-lateral application, with a reduction in the number of resolvers required, is disclosed and claimed in Case 7, SN. 595,702 filed July 3, 1956 by Robert W. Tripp for Bilateral Electrical Resolver System, now Patent 2,866,597, dated December 30, 1958.

FIGS. 5 and 6 show the portion of the system for position checking, the checking for X and Y being shown in FIG. 5 and for Z in FIG. 6. Referring to FIG. 12, the point 0 (X, Y, Z) is the present carriage position with respect to the X, Y and Z axes. The point 0 (X, Y, Z), may be taken as the origin, or its position may be identified by coordinates with reference to an origin located elsewhere. The slope of the out has the slope angle 0 to the X axis, and it has the slope out of the X, Y plane. The slope of the cut is extended to point AS on the vector S. If the tool were to follow the extended slope AS it would miss the assumed check point P (X Y Z on the desired generated curve. The invention provides for measuring the amount by which the tool would miss the check point P (X Y Z with provisions for correcting this error.

It will be seen that in the two dimensional case, somewhat as appears in FIG. 13, the points 0 and P identify a vector having the component As along the extended slope and having the component e at right angles to that slope. When the value As is Zero, the points 0 and P coincide and when the value e is small enough such as few hundredths of an inch, for example .02 to .00 inch, this event can be used to check the slope control and the curvature control. The orthogonal components of the vector between 0 and P are known for the following reasons. A value proportional to the X ordinate of the present workpoint O is represented by the X shaft output (shaft 122) from the differential gear DG5, acquiring this instruction from shaft S11 and the command data from the inputs D2 to D5. The X ordinate of the check point P is given in fine, medium and coarse increments by electrical signals in the lines 182, 183 and 184 respectively from the digital-to-analog linear converter 185 for the linear input data pertinent to X and indicated at D6, see FIG. 1. The fine signal in line 182 is fed to the quadrature windings 185 of the synchro resolver R3 and its output 187 contains a signal proportional to the difference between its electrical input in line 182 and its X shaft input, this signal being proportional to the difference between the X coordinate of point P and the X coordinate of point 0, or Ax as shown in FIG. 13. The medium resolver R4- and the coarse potentiometer 188 are also driven in accordance with the X shaft input 122, but through the lO-to-l gearing 189, as in the case of gearing 117 and 166 for X and gearing 124 (corresponding to 189), 123 and 175 for Y, in FIGS. 5 and 9 and also gearing 125 (corresponding to 189), 118 and 119 for Z in FIGS. 6 and 10. In a similar way, the output of the medium resolver R4 and the coarse potentiometer 188 also contain the Ax signal (the diiference between the X coordinates of the points P and O) in medium and coarse increments, these error signals and the fine error signal from R4 being supplied to the switch SW4 in circuit with one of the windings 190 of a four winding synchro resolver R6, see FIG. 5.

In a similar way, see FIG. 1, the Y coordinate of the check point P FIG. 12, is derived from the linear input D7, FIG. 1, having a digital-to-analog converter 191 which supplies the fine, medium and coarse increments of the Ay primary check data over the lines 178, 179 and 181), respectively as indicated to the fine resolver R7, FIG. 5, the medium resolver R8 and the coarse potentiorneter 192. The fine signal in line 178 is fed to the quadrature windings 167 of the synchro resolver R7,

FIG. 5, and its output 168 contains a signal proportional to the difference between its electrical input in line 178 (characteristic of the check point) and its Y shaft input (characteristic of the present tool position) of shaft 169 from differential gear DG6, this signal being proportional to the difference between the Y coordinate of point P and the Y coordinate of point 0 or Ay as shown in FIG. 13. The medium resolver R8, FIG. 5, and the coarse potentiometer 192 are also driven in accordance with the Y shaft input :169 but through the 10-to-1 gearing 124. In a similar way, the output of medium resolver R8 and the coarse potentiometer 192 also contain the Ay signal (the difference between the Y coordinates of the points P and O) in medium and coarse increments, these error signals and the fine error signal from R7 being supplied to the switch SW5 in 13 circuit with the other input winding 193 of synchro resolver R6, see FIG. 5.

Ax and Ay are constantly obtained and fed into the resolver R6 in the manner shown in FIG. 5. Resolvers R6 and R9 are set at the angle 6 by shaft 4. Resolver R6 has an output e which is an input to resolver R9, and in the two dimensional case, resolver R9 has outputs which are the correction values. Its outputs are here shown as AAx and AAy, for the three dimensional case considered a little later. As described later, when the distance As in the output of resolver R6 has decreased to a certain preselected value (a small fraction of the distance between two successive points such as O and P a switch closes which allows the correction to start. The correction values are added to the computed X and Y values respectively. The vector sum of these values is used to drive the carriage of the machine. When the carriage or tool position has been brought back to the required curve, the correction values in- .the output of resolver R9 will become'zero. The distance between two successive points on the curve such as O and P is limited to a value whereby the machine and computer combination will not be in error by values that exceed the allowable machining tolerances.

The three dimensional case will now be considered, first with regard to the solution and then with regard to the apparatus involved for axis Z, in addition to the apparatus described above for X and Y. FIG. 12 illustrates the problem. Point O is taken as the present position of the machine carriage. Its coordinates are X, Y, Z. This point is equivalent to point for the two-dimensional case. The relative motion between the carriage and the tool is indicated by the slant line S whose direction is defined by the angle 0 from the projection of this line on the X, Y plane to the X axis and the angle 3 to the X, Y plane. The carriage should proceed toward the point P but as it approaches P its position is displaced from P by the three compenents AAx, AAy, AAz. These are the errors to be found and corrected. The net resultant of these three mutually perpendicular errors is shown as e. It is (by definition) the shortest distance from point P to the line S extending from point P in the direction determined by the angles 0 and More specifically, the vector e is the miss distance from the point P if the carriage continues to travel without correction, along its present path defined by the angles 0 and c. When the coordinate errors AAx, AAy, AAz have been found, the carriage position can then be corrected as it approaches the point P These errors will be obtained in the followinganalysis:

Project a plan view (X, Y plane) of FIG. 12.

The coordinates Ax, Ay are measured in coordinate system 1 (see FIG. 13). Rotate this system about the Z axis through the angle 6. The new coordinates of the point P in this new rotated system are As and e, wherein e is the projection in the X, Y plane of the 3-D resultant error e. These values are obtained with resolver R6 (see FIG. 14).

Now looking at the y plane, it is seen that the point P is represented by the coordinates Az and As with respect to coordinate system 2.

Rotating this system through the angle results in coordinate system 3 (see FIG. 15). The point P in this system is represented by the coordinates AAz' and AS. The coordinate AS is the magnitude of the vector S shown in FIG. 12. The solution for these coordinates can be accomplished as shown in FIG. 16. The vector AAz' has components in all three mutually perpendicular directions of the original system X, Y, Z of FIG. 12. The value AAz of FIG. 15 is one of the three error values to be found. It can be seen as a component of AAz'. The other component of AAz in FIG. 15 is in the X, Y plane and is shown in the figure as the vector I. This 14 vector is in the plane of FIG. 19 and is perpendicular to the vector e. w

The coordinates AAz and l are obtained as shown in FIG. 17.

The resolver computations shown in FIG. 18 will obtain the coordinates AAx and AAy.

By combining FIGS. 14, 16, 17 and 18, the entire solution is presented in FIG. 22. In FIGS. 5 and 2.2, with resolvers R6 and R9 set at the angle 0 as shown, with resolvers R10 and R11 set at the angle 5, with Ax and Ay inputs to resolver R6 and with Az and As inputs to resolver R10, resolvers R9 and R11 supply the values AAx, AAy and AAz.

The value AS in the output'of resolver 10 is used in the same manner as is As in the two dimensional case. That is, where its value has decreased to a preselected value the error corrections start taking place.

POSITION CHECKER FOR Z In a manner somewhat similar to that described in connection with the input data pertinent to X and Y, the orthogonal components of the vector between 0 and P are also known with reference to the Z axis. The Z ordinate of the check point P is derived from the linear input D11, FIG. 2, having a digital-to-analog converter 59 which supplies the fine, medium and coarse increments of the Az primary check data over the lines 194, 195 and 196 rfiSPGCfiVely. The fine signal in line 194 is fed to the quadrature windings 197 of the synchro resolver R12 and its output 171 contains a signal proportional to the difference between its electrical input in line 194 (characteristic of the check point) and its- Z shaft input (characteristic of the present tool position) of shaft 172 from dilferential gear DG7, this signal being proportional to the difference between the Z coordinate of point P and the Z coordinate of point 0 or Az as shown in FIG. 15, The medium resolver R13 and the coarse potentiometer 170 are also driven in accordance with the Z shaft input 172 but through the lO-to-l gearing 125. In a similar Way, see FIG. 6, the output of medium resolver R13 and the coarse potentiometer 170 also contain the Az signal (the difference between the Z ordinates of the points P and O) in medium and coarse increments, these error signals and the fine error signal from R12 being supplied to the switch SW6 in circuit with the input winding 173 of synchro resolver R10.

It will be apparent then that as shown in FIGS. 5 and 6, resolver R6 has inputs of Ax and Ay, and outputs of e and As. Resolver R10 has inputs of As and Az and outputs of AAz' and AS. Resolver R11 has an input of AAz and outputs of AAz and l, the latter and e being inputs for resolver R9 which has as outputs AAx and AAy. Also, resolvers R6 and R9 are set at angle 0 by shaft 4 from diiferential gear DG1 in FIG. 1, resolvers R10 and R11 being set at the angle by shaft 5 of differential gear DG3 in FIG. 2.

The value AS in the output of resolver R10, FIG. 6 is supplied to relay A and also to relay B. RelayA has a contact 72 in circuit with the AAz output of resolver'Rll and with amplifier 73 and motor M3, having a shaft 74 whereby the instruction of Az in shaft S13 is added by differential gear DG7 to the instruction in shaft 74 to form the z shaft drive 172. Relay A has a contact 75 in circuit with the AAy output of resolver R9 and with amplifier 76 and motor M4 having a shaft 77 whereby the instruction of Ay in shaft S12 is added by differential gear DG6 to the instruction in shaft 77 to form the shaft drive 169. Relay A also has a contact 78 in circuit with the AAx output of resolver R9 and with amplifier 79 and motor M5 having a shaft 80 whereby the instruction of Ax in shaft S11 is added by differential gear DGS to the instruction of shaft 80 to form the x shaft drive 122, see FIG. 5.

As shown in FIG. 1, relay A of FIG. 5 also has a switch contact S10 which when open, opens the circuit of servo motor M2, and when closed, closes this circuit. 

